1. Field of the Invention
This invention is directed to a method and apparatus for remotely monitoring with high reliability a process carried out in a hazardous environment within a containment structure. More particularly, it is directed to a method and apparatus for transmitting redundant signals from a large number of detectors through the containment structure with a minimum number of penetrations. It is particularly applicable to transmitting to the plant control room outside of the containment building reliable indications of control rod position in a nuclear reactor.
2. Prior Art
There are many installations wherein the hazardous conditions under which a process is carried out require that it be enclosed in a containment structure. Under such conditions, it is desirable to be able to remotely monitor and control the process and to be able to do so with a great deal of reliability to preclude having to shutdown for lack of sufficient control, and with a minimum of pentrations through the containment structure to reduce construction costs. The latter requirement becomes particularly important in installations in which a large amount of information must be transmitted through the containment structure.
Radiation and toxic chemicals are exemplary hazardous conditions which require confinement within a containment structure to protect those working with them. A nuclear reactor is an example of an installation in which hundreds of detectors and control signals of various kinds must be transmitted through the containment structure for monitoring, protection, and control functions directed from outside. Remotely monitoring the position of the control rods within the core of the nuclear reactor is a procedure which requires the transmission of a great deal of information through the containment structure.
In a typical pressurized water reactor, there are several scores of drive rods which each incrementally step several neutron absorbing rods connected by a spider into and out of the reactor core. The positions of the neutron absorbing rods within the reactor core are derived from the positions of the associated drive rods; each of which is equipped with a position detector. Early rod position detectors used the change in impedance of a coil or the change in coupling between primary and secondary coils effected by the end of the drive rod as it moved through the coil to generate an analog signal indicative of rod position. The susceptability of such analog systems to variations in temperature, rod magnetization, permeability of the rod, power supply voltages and frequency, and to interference from adjacent control rods and drive mechanisms, led to the development of digital position indicating systems such as that disclosed in U.S. Pat. No. 3,846,771. This system uses a number of discrete coils spaced along the linear path traced by the end of the rod. The change in impedance of each coil in sequence as the rod advances through successive coils is used to generate discrete output signals or one output signal with discrete levels. Discrete output signals are preferred over a single signal with discrete levels because they can be more easily transmitted to remote locations and reliably decoded. Their use, however, significantly multiplies the number of signals that must be transmitted.
Reliability is of critical importance in monitoring the position of control rods in the core of a nuclear reactor. It is well-known that reliability can be enhanced through redundancy. In the digital rod position indicating systems disclosed in U.S. Pat. Nos. 3,858,191 and 3,893,090, redundant sets of coils are interleaved along the path of each drive rod and the signals generated as the end of the rod passes through the coils in each set are separately transmitted through the containment building wall through separate channels to the outside where they are combined to provide an indication of rod position. All of the data generated by one set of coils in each detector is transmitted over one channel and that generated by the second set of coils in each detector is transmitted over another channel. If either channel is inoperative, the remaining channel provides the position indication for all of the rods with half the resolution of the combined indication. While such systems provide protection against a single component failure, they are highly susceptible to certain dual component failures. For instance, if one transmission channel should fail, the signals from one entire set of coils for each detector are not available. A subsequent failure in the other set of coils for any detector results in a loss of all reliable information regarding the position of the neutron absorbing rods connected to the associated drive rod.
It is a primary object of the present invention to provide the capability for remotely monitoring a process carried out in a hazardous environment within a containment structure with high reliability and with a large amount of digital information transmitted using a limited number of penetrations through the containment structure.